A large fraction of the energy consumption of the world is utilized to generate electrical power. However, two of the major methods of generating that power, nuclear fission and combustion of fossil fuels, have significant problems.
The major problems of fossil fuel based electrical power generation include: (1) the earth's limits to fossil fuel supply, and (2) the generation of the greenhouse gas carbon dioxide from the oxidation of fossil fuels.
The major problems with generating electrical power by nuclear fission are often cited to be: (1) disposal of hazardous wastes and (2) potential issues of plant safety.
Generation of electrical power directly from sunlight via solar cells offers great potential as a source for the world's future energy supply and does not have the problems noted above.
However, the current higher cost (compared to fossil fuels and nuclear fission) of electrical power via solar cells precludes its use in many applications. A major opportunity exists for new technology combining lower cost materials and more environmentally friendly processes for making solar cells which would result in significantly lower cost electrical energy.
A further need for more effective energy usage from solar cells requires the storage of energy for use in periods of very little to no sunlight.
As used herein, the electromagnetic radiation from the sun incident upon the surface of the earth is defined as the solar spectrum. This spectrum has a wavelength distribution from about 2000 to 800 nanometers (the near infrared range) through 800 to 400 nanometers (the visible range) and a small amount from 400 to 300 nanometers (the ultraviolet range). As used herein, photovoltaic is a term describing a structure which absorbs radiation of the solar spectrum and directly produces voltage and current. As used herein, a solar cell is a current and voltage producing device which includes a structure having photovoltaic capability.
From a broad perspective there are three major requirements for a useful photovoltaic component of a solar cell:
1. A material which absorbs, in a practical thickness, a useful amount of radiation of the solar spectrum.
2. Properties of this material and an arrangement relative to at least one second material providing a mechanism whereby this absorbed radiation produces negative and positive charges which can be separated.
3. A pathway through which the separated negative and positive charges produce current and voltage at useful levels which can be utilized in an external circuit.
In current solar cell technologies the components providing the photovoltaic effect can be arranged in several different geometries (Stephen T. Thornton and Andrew Rex, Modern Physics for Scientists and Engineers, 2nd ed., 1993, p. 419). One of the well-known arrangements of materials utilizes a metal joined to a semiconductor forming a junction known as an MS junction. This interface between the metal and the semiconductor is termed a Schottky barrier junction. A variation of this arrangement is one in which an electrical insulator is inserted between the metal and semiconductor forming an MIS junction. Both of these material arrangements have limitations in cost and/or performance which limit their usefulness in solar cells.
The predominant solar cell technology in the current market is based on the semiconductor crystalline silicon. In this technology thin wafers are made in which one side of the wafer is doped with a material to provide an excess of negative charge carriers (an n-doped material) and the other side of the wafer is doped with a material to provide an excess of positive charge carriers (a p-doped material) so that the structure forms a p-n junction. The p-n junction is an interfacial boundary which leads to the electrical asymmetry to separate the positive and negative charges upon the absorption of light (a requirement as noted above).
This silicon based technology has some severe limitations. The relatively small silicon wafers are made in a batch process which requires major capital investment cost per unit of production (Christoph Brabec, Vladimir Dyakonov, and Ullrich Scherf, eds., Organic Photovoltaics: Materials, Device Physics, and Manufacturing Technologies, Wiley-VCH, 2008, p. 553). This approach has major difficulties to reach the very large volume production at low costs which will be required to make electricity from solar cells on very large scales. Some estimates place the potential for electricity from solar cells to require an installed area of 100,000 km2 in fifty years.
Among the different configurations and materials for making solar cells is a more recently used design based on conducting polymers or a conducting polymer and a spherical fullerene compound derivative. This more recent technology utilizes organic electron donors and electron acceptors which are intimately blended in a single layer and show electron transfer upon absorption of radiation of the solar spectrum. This technology which is frequently referred to as Organic Photovoltaics (OPV) holds promise for some reduced cost and more manufacturing flexibility. For example, it is much more amenable to roll to roll high volume production in sheet form utilizing printing technology.
However, OPV technology also has several limitations. These devices frequently use materials which at this time are made by processes which are not well developed at the scales necessary for low cost production. Furthermore, the processes for making the organic films for these technologies rely on organic solvents. These solvents (while safer than many) pose significant potential environmental impacts when used at the scales which will be necessary to make very large volumes of these devices.
Furthermore, some of these organic materials (polymers and lower molecular weight organic molecules) are susceptible to degradation under the action of radiation of the solar spectrum and chemical degradation from atmospheric oxygen.